The present invention relates to optical image acquisition devices. More particularly the present invention relates to image pickup modules adaptable for scanning fingerprints, handprints, and other biometrics.
Image pickup devices and modules are being implemented in increasing numbers as more and more images are digitized for storage and processing. For example, biometric devices such as fingerprint scanning devices are used in increasing numbers as identifying devices used to identify a user for the purposes of providing access to restricted information, equipment, or services.
FIG. 1 illustrates one embodiment of a prior art image (fingerprint) pickup system 10. In the prior art system 10, a light source 12 provides incident light 14 that is transmitted into a prism 16 through a light receiving surface 18 of the prism 16. The incident light 14 meets an imaging surface 20 at an incident angle 22 with respect to a normal angle to the imaging surface 20, the normal angle represented by a normal line 24. At the incident angle 22, the incident light 14 experiences total internal reflection (TIR) at the imaging surface 20 because the incident angle 22 is greater than the critical angle with respect to the imaging surface 20. The TIR phenomenon and the critical angle are known in the art.
As the incident light 14 reflects off of the imaging surface 20, different portions of the incident light 14 experience different amount of scattering, absorption, reflection, or a combination of these. This is because of fingerprint ridges and valleys present at the imaging surface 20. Accordingly, the reflection results in reflected light 26 having portions of varying intensities, the varying intensities representing the fingerprint pattern.
The reflected light 26 is then focused by a lens assembly 28 onto an image sensor 30 which converts the reflected light 26 including the fingerprint pattern it embodies into electronic signals to be processed and stored by a processor 32 connected to the image sensor 30. Vectors 14 and 26 and other vectors used in the Figures to illustrate directions of light are used only for clarity of discussion and are not intended to represent ray traces as is often used in the art of optics. For simplicity, the lens assembly 28 is illustrated as only one lens; however, the lens assembly 28 may include multiple lens, optics components, or a combination of these.
To focus the reflected light 26 onto the image sensor 30 with minimal distortion, the lens assembly 28 and the image sensor 30 are often placed relatively far from the prism 16 including the imaging surface 20. For example, U.S. Pat. No. 3,975,711 granted to McMahon specifies 40 centimeters (cm) for the distance between the prism 16 and the lens 28.
To reduce bulk and manufacturing costs, it is often desirable to reduce the height 34 of the image pickup system 10. However, moving the lens assembly 28 and the image sensor 30 closer to the prism 16 would introduce and increase distortions of the captured image (fingerprint pattern in the present example). Further, to place the lens assembly 28 closer to the prism 16, a larger lens assembly 28 would be required compared to the required size of the lens assembly 28 placed relatively distant from the prism 16. This is to reduce distortions of the image (fingerprint pattern) aggravated by the closer placement of the lens assembly 28, especially near the edges of the image (fingerprint pattern). In the illustrated sample, the height 34 can also be considered the focal length required to effectively focus the image (fingerprint pattern) onto the image sensor 30
Currently, image pickup modules having a height 34 ranging from four to five cm are available. However, even these modules are too tall and bulky to be conveniently implemented in smaller electronic devices such as a mobile wireless telephones.
Consequently, there remains a need for a more compact image pickup module and system.